Carbon sequestration in aggregates from native and cultivated soils as affected by soil stoichiometry

Quantitative influence and underlying mechanisms of nutrient stoichiometry on mineralization of native Soil organic C (SOC) and straw C in different aggregate classes from cultivated and non-cultivated soils are still unclear. Soil samples (Mollisols) from a native woodlot and a farmland converted from woodlot were sieved into three aggregate classes (mega-aggregates (6.3-2 mm), macro-aggregates (2-0.25 mm), and micro-aggregates (< 0.25 mm)) and incubated (180 days) under different nutrient rates (nil, low, and high supplies of N and P) with or without 13C-enriched straw amendment. Significantly higher percentage of native SOC was mineralized from mega- and macro-aggregates (65.8-82.2%) compared with micro-aggregates (48.3-52.0%) in woodlot soil. Nutrient addition significantly increased aggregate-associated C in both soils with straw, and the increase was greater in farmland than in woodlot soil and in large-sized aggregates than in micro-aggregates. These results suggested that large aggregates serve as a C reservoir of labile C, while micro-aggregates that contained C was not easily mineralized even with abundant nutrients. Differences of C mineralization among aggregate size classes were significant in woodlot soil but not in farmland soil. Depletion of SOC was greater with increasing nutrient addition rates in farmland aggregates without straw, while the depletion in woodlot aggregates showed no difference among nutrient treatments, suggesting that microbial activity was nutrient-limited in farmland aggregates. The results improved our knowledge on SOC mineralization in response to residue-nutrient management in different aggregate classes from cultivated and non-cultivated soils, which have important implications for strategies to improve soil fertility or mitigate climate change via increased SOC.